Thin film deposition on a substrate for semiconductor device fabrication and other applications is frequently accomplished through a gas phase process using a gas/vapor mixture containing the precursor vapor needed for film formation. The mixture is usually introduced into a deposition chamber under suitable temperature and pressure conditions to form a thin film on the substrate. In the case of a precursor in liquid form, the precursor vapor can be generated by heating the liquid to a suitably high temperature. A carrier gas can then be bubbled through the liquid to saturate the gas with vapor to form the desired gas/vapor mixture. Alternatively, vapor can be generated by injecting the liquid directly onto a hot metal surface to vaporize the liquid and form vapor. At the same time, a carrier gas is also injected to carry away the vapor to produce the gas/vapor mixture. In recent years, liquid vaporization through direct liquid injection and droplet vaporization is increasingly used. In this process, the precursor liquid is injected into an atomization apparatus with a carrier gas to form a droplet aerosol comprised of small droplets suspended in the gas. The droplet aerosol is then heated to form a gas/vapor mixture in a heated vaporization chamber.
Precursor vaporization by atomization followed by droplet vaporization in the carrier gas has the advantage that droplets are vaporized while suspended in the gas. Heat is transferred indirectly from the heated vaporization chamber walls through the gas, then into the suspended droplets for vaporization. Direct contact between the liquid and a hot metal surface can be eliminated. Contact between the precursor liquid and a hot metal surface can cause the precursor to thermally decompose to form undesirable by products. Droplet vaporization can greatly reduce thermal decomposition to produce a high purity gas/vapor mixture to form thin films in semiconductor device fabrication. In addition, due to the evaporative cooling effect, the surface temperature of an evaporating droplet remains low, further reducing thermal decomposition that can occur in the liquid phase at sufficiently high temperatures.
While droplet vaporization has been used successfully in recent years to vaporize precursor chemicals for semiconductor device fabrication, many modern precursor chemicals are difficult to vaporize. The problem of thermal decomposition and by-product formation has remained as a result of design shortcomings in the liquid atomization apparatus. This is particularly true for high molecular weight precursors with a low vapor pressure. Such low vapor pressure precursors typically have a molecular weight higher than approximately 300. Their vaporization requires the use of comparatively high vaporization temperatures. Yet, these precursor chemicals are less stable and prone to thermal decomposition that can form by-products that are harmful to the semiconductor device being fabricated.
When liquid is introduced into a heated vaporization chamber through an atomizer, the small liquid flow passageway usually must pass through a high temperature region in which the liquid passageway itself becomes heated. Over time, decomposition products can form and accumulate in the small, heated liquid flow passageway and cause the passageway to become clogged. The accumulated decomposed material in the liquid flow passageway can also be dislodged and appear as a gas-borne contaminant in the gas/vapor mixture. These contaminants can be carried by the gas/vapor mixture into the deposition chamber and deposit on the substrate surface to contaminate the substrate. The result is increased surface particle count on the product wafer, and increased defects in the device, and the loss of product yield.